Fiber optic cables are used today for carrying data as light pulses between various parts of a computer system. For example, fiber optic cables can link a plurality of host computers together, or link a host computer to one or more peripheral devices, or link a plurality of peripheral devices together. Cable lengths vary from a fraction of a meter to 30 or more meters. Advantages of fiber optic cables are their small weight and size and immunity to electromagnetic interference.
Date is placed on the cable by triggering a light source such as a light emitting diode placed in close proximity to one end of the fiber optic cable and attached thereto by a suitable connector. A light sensitive device such as a photodetector diode is positioned adjacent to the other end of the cable and also held in position by a suitable fiber optic connector. The photodetecting diode provides a small current output in response to light pulses received. The strength of the current output is proportional to the light received but in all cases this signal must be amplified by subsequent preamplification before further processing by the receiving circuitry to extract the data.
Commercially available, low cost fiber cables have an attenuation of approximately 0.6 db per meter. A 30 meter (100 foot) section has an attenuation loss of 18 db while a 3 meter (10 feet) section has an attenuation loss of only 1.8 db. Therefore, the power variation in cable attenuation between a 3 meter section of cable and a 30 meter section is 16 db which is equivalent to a 32 db variation in voltage. This means that the receiver amplifier section sees a dynamic range of 32 db in voltage which is too large a voltage variation for it to handle without the use of complicated automatic gain circuits (AGC). Even if the attenuation of the cable is reduced, applications requiring much longer runs of cable are sure to occur whereby wide variations in attenuation between cables of varying length becomes a problem.